Intestinal Parasites in Horses

What are intestinal parasites?

Parasites differ from other organisms living in or on a host in that they are ultimately detrimental to the health of the host. Common internal parasites of horses include worms (helminths), insects (arthropods), and microscopic organisms (protozoa). Intestinal parasites affect horses more frequently than respiratory, musculoskeletal or central nervous system parasites.

Although different parasite species have different life cycles, the basic pattern involves eggs hatching, developing into larvae, migrating through the body of the host (which often causes the most damage), and maturing into adults that lay hundreds to millions of eggs per day. Horses usually become infected with the eggs or larvae when they eat grass, hay, or grain contaminated with manure.

A large variety of parasites can infect horses, but the intestinal parasites that are the most common and present the greatest health risks include:

• Large strongyles (Strongylus spp.) - Also known as bloodworms or red worms, this species is one of the most harmful parasites of horses. Eggs in manure hatch into larvae that horses consume when grazing. The larvae mature in the intestinal tract and burrow out into blood vessels. From there, they move through different organs and eventually migrate back to the intestine. The larvae’s movement can cause significant damage to the lining of blood vessels, leading to the formation of blood clots. The clots may detach and be carried in the bloodstream, eventually lodging, blocking blood flow, and resulting in tissue death. Infection rates are usually highest in the spring and summer when weather conditions favor the development of larvae on pasture. Large strongyles have become less common in horses due to effective deworming programs.
• Small strongyles (cyathostomes) - The life cycle of small strongyles is similar to large strongyles in that eggs in manure hatch into larvae that can be consumed by grazing horses. However, unlike large strongyles, small strongyles do not migrate outside of the intestinal tissues. The larvae of small strongyles may become encysted (enclosed as if in a cyst) in the large intestine by burrowing into the intestinal wall and lying dormant waiting for the proper conditions to emerge. During this encysted period, small strongyles larvae are not susceptible to most dewormers. Infection levels tend to be greatest in the late winter and early spring. Even with the increased use of dewormers, small strongyles remain a concern in most equine populations.
• Ascarids (Parascaris equorum) - These large roundworms affect young (< 1 year of age) horses far more often than mature horses. Consequently, young horses are the primary source of the infective stage of the parasite (the egg) in the environment. The 6- to 12-inch-long worms can number in the hundreds in the horse’s small intestine and can adversely affect its nutrition. Ascarids may also cause blockage of the intestine or migrate through the lungs.
• Stomach worms (Habronema spp.) - Fly larvae in the manure of horses ingest the larvae of the stomach worm. The fly matures and deposits the worm larvae in the lips of horses, or horses may ingest the fly. Larvae in the stomach mature into adult worms and lay eggs that are passed in the feces. Far more serious than the stomach worms themselves are the lesions resulting from deposition of larvae at sites where flies sometimes feed, such as the corner of the eye or in a skin wound. The ensuing inflammatory response is characterized by fleshy masses that bleed easily - a condition known as cutaneous habronemiasis or summer sores.
• Tapeworms (Anoplocephala spp.) - The most common species of tapeworm found in horses is Anoplocephala perfoliata. Mites living in a horse pasture may consume tapeworm eggs from the feces of infected horses. Horses can then swallow the mites while grazing, and consequently become infected with tapeworms. The mites thrive in areas with green pastures and are uncommon in dry, arid regions. Once ingested, tapeworms attach to the intestinal wall at the intersection of the small and large intestine, which can cause colic.
• Pinworms (Oxyuris equi) - Though less dangerous than other internal parasites, pinworms are annoying to the horse because they cause intense itchiness of the region around the anus - the perineum. Horses acquire the parasite by consuming contaminated water or feed or by licking the sticky eggs off walls or fences. Worms mature in the large intestine over 3 to 4 months and deposit their eggs on the surface outside the anus. The eggs hatch outside of the horse’s body and become infective in a few days, although they can survive unhatched for several months.
• Bots - Bots are the larvae (immature flies) of the botfly (Gasterophilus spp.). Since these flies are common in the horse’s environment, it is likely that most horses will become infected at some point in their lives. During the late summer and early fall, adult botflies lay eggs on the hair of various parts of horses, particularly around the chest, forelegs, throat, and nose. Stimulated by the horse’s licking, the larvae hatch and enter the horse’s mouth, settling in the tissues of the gums, cheek and tongue. After a month, the larvae migrate and attach to the stomach lining, causing irritation and interfering with digestion. Bots are infrequently associated with disease, but can be unsightly.

What are the clinical signs of intestinal parasites?

Clinical signs of internal parasites in horses can vary based on the type(s) of parasites present. Diarrhea, weight loss, fever, and poor coat condition are common clinical signs associated with large and small strongyles. Large strongyles can cause inflammation of the abdominal wall membrane (peritonitis) and severe colic in cases where tissue death leads to inadequate blood supply (infarction). Weight loss and intermittent colic are more commonly observed with small strongyles infection.

Colic, coughing and diarrhea are also common clinical signs associated with ascarid infection, along with poor growth, rough hair coats, chronic respiratory problems, pneumonia, and sometimes death.

Heavy tapeworm infections may cause weight loss, diarrhea and occasionally colic due to partial occlusion of the ileocecal orifice (the point at which the small intestine joins the large intestine) or rupture of the cecum wall.

A characteristic of pinworm infection is rubbing of the tail and perineum, causing broken tail hairs and bare patches around the tail. However, some infected horses do not show any clinical signs.

How is a horse diagnosed with intestinal parasites?

It is often difficult to detect early parasite damage and even more difficult to establish a link to the parasites themselves. Diagnosis is generally based on finding parasite eggs in the manure. Two important diagnostic tests are often performed together to provide information about the type and quantity of parasites present. Both tests are run on fresh manure.

• Fecal flotation - This test examines the different parasite eggs that might be present in a fecal sample at high magnification. The characteristic egg type can identify the broad group of parasite present (for example, strongyles).
• Fecal egg count (McMasters) - This test uses a dilution technique to count the number of eggs per gram of manure. If the parasite infection has proceeded to the point in the life cycle where the adult worms are producing eggs, then this test allows an estimation of the worm burden in an individual horse. This test reports the shedding status of the individual horse at the time of sampling. It does not accurately measure the total parasite burden of that horse. It also does not detect parasites in their larval stages.

If a veterinarian suspects that the parasite population on a property has developed resistance to a specific deworming agent, fecal egg count testing can be performed before and after treatment to assess the effectiveness of the treatment. To perform the fecal egg count reduction test, a fresh manure sample is analyzed the day the horse is dewormed, and a second sample is analyzed 14 days later. If the egg count is not decreased by more than 90%, resistant parasites are likely present in the horse and the environment.

It should be noted that levels of strongyle egg shedding are extremely variable among mature horses (over 3 years of age), but are generally stable over time within otherwise healthy individuals. Consequently, low shedders will reliably have low fecal egg counts (<200 eggs per gram of manure) and high shedders (>500 eggs per gram of manure) will tend to have high fecal egg counts, although studies have reported that some horses may switch categories if they are near the cutoff values. Low shedders make up approximately 50-75% of the adult equine population. High shedders are present in 10-30% of the population, and the remaining 5-15% are considered moderate shedders. The underlying mechanism for these differences is likely genetic and research is ongoing. The Pioneer 100 Horse Health Project aims to discover genetic differences that could lead to certain horses always being low or high shedders.

What is the treatment for intestinal parasites?

It is important to choose the proper FDA-approved medications for the intended use and time treatments to control egg shedding in the environment. The AAEP recommends one to two treatments per year for all adult horses as a basic foundation. Treatments for ascarids and pinworms are usually given on an “as needed” basis.

Several anthelmintic medications are available against migrating larvae and adult worms, but others are only effective against adult worms. Frequent use of anthelmintics in recent decades has resulted in high levels of anthemintic drug resistance, particularly against pyrantel, fenbendazole, and oxibendazole.

Current deworming recommendations include:

• Ivermectin and moxidectin are the foundational treatments for control of strongyles.
• Praziquantel, or a double dose of pyrantel pamoate, given in fall or early winter is recommended for tapeworms, as needed.
• Ascarids have developed resistance to ivermectin, moxidectin, pyrantel, and benzimidazole. Benzimidazoles and pyrantel salts are the best choice for Parascaris treatment. Fenbendazole may be used against larvae. Benzimidazoles include fenbendazole (Panacur®) and oxibendazole (Anthelcide®EQ).
• Pinworms have developed resistance to ivermectin, so benzimidazoles are currently the best treatment choice. Washing under the tail can also relieve clinical signs.

Bots may be treated with a boticide once a year in the late fall or early winter to decrease transmission. Ivermectin and moxidectin may be used for treatment against bots, but moxidectin may have variable effectiveness depending on the life stage of the bot.

A general deworming plan based on fecal egg count results is below:

*EPG = eggs per gram of manure

• Products containing ivermectin include Equell®, Zimectrin®, Rotectin®, and IverCare®. Products containing ivermectin with prazinquantel include Equimax® and Zimectrin Gold®.
• Products containing moxidectin include Quest®, and others. Quest Plus® contains both moxidectin and prazinquantel.

Natural and organic remedies have not been shown to be effective for managing equine intestinal parasites.

What is the prognosis for horses that have intestinal parasites?

In general, internal parasite infections in horses are relatively mild and only cause disease at very high levels. Heavy internal parasite loads can cause significant health effects and, in rare cases, death.

How can intestinal parasites be prevented?

Traditional parasite control programs based on rotational treatments are no longer recommended. The American Association of Equine Practitioners (AAEP) updated their Internal Parasite Control Guidelines in 2024.

Parasites will eventually find their way into the intestines of almost every horse. Many stabling/housing situations perpetuate high parasite loads by crowding horses together on pasture, thereby limiting their range and allowing their food and water to be contaminated by egg- and larvae-infected manure.

An effective parasite control program tries to decrease the number of mature adult parasites and, in turn, the number of eggs shed. This can be achieved in a three-step strategy:

1. Decrease the number of adult parasites laying eggs by deworming tailored to your particular situation.
2. Decrease the number of larvae and eggs eaten by a horse by removing manure from feeding areas. The pickup and disposal of pasture manure yields dramatic results and longer intervals between deworming. This is an important benefit considering the increase in drug resistance by parasites. Clean stalls decrease fecal contamination of food and water.
3. Evaluate the effectiveness of the first two controls twice a year. Periodic FEC surveillance is essential to controlling parasites as an individual horse’s shedding status may change due to changes in immune status and exposure to various parasites.

The goal of parasite control programs in horses is not to eliminate all parasites from an individual or population. This would be impossible, and attempts toward this goal would rapidly lead to parasite drug resistance. Rather, the goal is to limit parasite infections to keep horses healthy and free from disease.

For more information

AAAEP Internal Parasite Control Guidelines (updated May 2024)

Internal parasite testing for horses through the California Animal Health and Food Safety Laboratory (CAHFS)

Internal parasite testing for horses through the UC Davis veterinary hospital clinical laboratory services